This invention relates to the field of corrosion-resistant alloys and more particularly to low strategic metal content workable alloys resistant to both oxidizing and reducing sulfuric acid solutions over a wide range of acid concentrations.
For purposes of analyzing and predicting their corrosive effect on various metals, acids and other corrosive agents are commonly classified as either "oxidizing" or "reducing." A reducing medium is one in which the strongest oxidizing agent is the hydrogen ion or hydronium ion while an oxidizing medium includes components which are more highly oxidizing than either the hydrogen ion or hydronium ion. Sulfuric acid is normally a reducing acid but high strength sulfuric acid is often oxidizing, especially at elevated temperatures. Moreover, various industrial sulfuric acid streams contain various oxidizing acids and salts as contaminants. It is, therefore, desirable that an alloy designed for general utility in industrial sulfuric acid streams be resistant to both reducing and oxidizing environments.
Corrosion resistance of any given metal or alloy in a reducing medium is often sharply different from its resistance in an oxidizing medium, with some metals and alloys being more resistant to reducing media and others to oxidizing media. These differences in behavior are thought to be attributable to differences between the corrosion mechanism in a reducing medium and the corrosion mechanism in an oxidizing medium. Thus, corrosive attack by a reducing acid is generally considered to involve attack on the metal by hydrogen ions resulting in the oxidation of metal to soluble ions and release of hydrogen gas. Metals of relatively high nobility, therefore, as indicated by their positions in the galvanic series, are generally resistant to corrosion by reducing acids. Attack by oxidizing media on the other hand does not involve release of hydrogen but commonly results in the formation of metal oxides or other metallic compounds at the metal surface. Unlike the situation with reducing acids, a favorable position relative to hydrogen in the electromotive series provides no insurance that a metal will not be rapidly attacked by an oxidizing medium. However, certain elements such as chromium, aluminum and silicon form tough insoluble oxide films on initial contact with an oxidizing medium and such films serve as barriers against further reaction between the medium and the metal, thus preventing further corrosion from taking place.
Sulfuric acid solutions are not only very corrosive generally but the nature of their corrosive properties varies markedly with both acid concentration and temperature. This variability relates at least in part to sulfuric acid's ambivalent assumption of both reducing and oxidizing properties as its concentration, temperature, and the nature and proportions of various contaminants are altered. As a consequence of this variability in its corrosive properties, few materials are available which are reasonably resistant to sulfuric acid solutions over a wide range of concentrations and temperatures. A relatively large number of available materials exhibit reasonable resistance to either dilute sulfuric acid solutions having an acid strength of less than about 20% by weight or to concentrated solutions having an acid strength greater than 80% by weight. A lesser number of materials are effective for the intermediate and generally more corrosive concentration range of 20-80%, and even fewer metals are commercially useful in contact with sulfuric acid solutions ranging from strengths below 20 to greater than 80%, particularly when exposed to elevated temperatures.
Of the known alloys which are demonstrably effective over wide ranges of sulfuric acid concentrations, many contain relatively high portions of nickel and chromium and are thus rather expensive. There are some known alloys which have no chromium or relatively low chromium contents, but these typically contain from about 16 to 32% molybdenum and up to about 5% tungsten, with less than 7% iron.
Parr U.S. Pat. No. 1,115,239 discloses the first known alloy containing nickel, chromium, molybdenum and copper, a combination now well recognized to be especially resistant to a wide range of sulfuric acid concentrations as well as to many other corrosive media.
LaBour U.S. Pat. No. 2,103,855 recognizes the effectiveness of silicon additions to such alloys in reducing corrosion, but at a drastic loss in ductility, workability and weldability. Silicon, a non-metallic element, has long been used in these alloys to increase hardness, wear resistance, and some ranges of corrosion resistance, but no acceptable way has been discovered to adequately counteract silicon's embrittling effect.
German Pat. No. 304,126 describes the austenitic alloys of about 18% chromium and 8% nickel content, known as the "18-8" stainless steels. Apparently Nekhendze of U.S.S.R. was the first to report on additions of both molybdenum and copper to "18-8" stainless steel in 1931. Thus began a series of iron-base alloys containing nickel, chromium, molybdenum and copper which exhibited advantageous corrosion resistant qualities, but did not equal the more expensive nickel-base alloys.
Research workers for many years have sought to gain the maximum corrosion resistance of nickel-base alloys, such as stainless steel, with the least amount of enrichment by critical alloying metals, i.e., the relatively expensive nonferrous metals which impart improved corrosion properties to the alloy.
One significant development in this series of alloys is described in Parsons U.S. Pat. No. 2,185,987, disclosing what came to be known as Durimet 20, Carpenter 20 or simply Alloy 20, of nominal composition 29% nickel, 20% chromium, 2.5% molybdenum, 3.5% copper, all weight percents, and the balance substantially iron. Alloy 20 has proven to be a standard of comparison against which later alloys are gauged. It possesses a desirable combination of moderately good general corrosion resistance, fine workability, and relatively low strategic alloy content. In terms of cost and relative availability, the elements that are most widely encountered in this family of alloys range as follows in order of increasing cost and decreasing availability: iron, silicon, manganese, copper, chromium, nickel, molybdenum and niobium. Tantalum may substitute for niobium in most cases but at increased cost.
A good deal of work has been done in alloys of this type with the objective of increasing hardness or precipitation hardness. Additional work has been directed to equaling the corrision resistance of Alloy 20 with leaner alloys (alloys of relatively lower critical alloy metal content) or improving upon the resistance of Alloy 20 with the least increase in strategic (critical) alloy metal content. Post U.S. Pat. No. 2,553,330 recognizes the improvement in workability of most types of corrosion resistant alloys brought about by minor additions of cerium or other components of misch metal. Other workers have noted improvements in workability often realized through minor additions of titanium, boron, nitrogen, and niobium either separately or in combinations under certain circumstances.
Scharfstein U.S. Pat. No. 3,168,397 describes alloys exhibiting generally improved resistance to corrosion by sulfuric acid and to stress corrosion cracking. This alloy is somewhat higher in strategic metals than Alloy 20 and nominally contains 32.5% nickel, 20% chromium, 2.3% molybdenum, and 3.3% copper together with one or more of misch metal, niobium, nitrogen, titanium and boron. This alloy is known as Carpenter 20Cb3 and contains about 38% iron compared to about 44% iron in Alloy 20.
Culling U.S. Pat. No. 3,759,704 describes nickel-base alloys of somewhat better general resistance to sulfuric acid solutions than prior nickel-base alloys, and notable for achieving this with increased chromium and reduced nickel contents compared to prior alloys. However, these alloys contain only 4 to 16% iron.
Culling U.S. Pat. No. 3,893,851 maintains a high chromium content but raises nickel to a maximum for increased workability. The alloy of this patent contains only 4% iron.
Culling U.S. Pat. No. 3,844,774 effects reductions in nickel and chromium contents as compared to U.S. Pat. No. 3,759,704, while raising iron to about 25%.
Culling U.S. Pat. No. 3,947,266 describes alloys in which iron is further increased to about 30% without losing sulfuric acid corrosion resistance. However, in view of the increasing scarcity and cost of strategic metals, many of which are imported, there remains the desirability of further reducing strategic metal content without sacrificing corrosion resistance or workability.